The
CD57 is the cheaper brother of the 'regular' CD67. The 57 has no
HDAM circuit and no optical digital output, and some electrolytics are
smaller. However,
the PCB and
transport are identical to that of the CD67, apart from the small
differences between the components. But those are going
to be removed anyway... So a CD57 is an interesting starting point for
modifications:
in the end, the result will be as good as with a 67, certainly with the
discrete output stage that is presented here. With this, it has turned
from a simple player into a real CD57-XXL.

In
this player the entire analog section after the DAC (the filter and the
opamps) has been removed. It's been replaced by a passive 3rd order
filter, followed
by a discrete output stage*. It consists of a differential stage that
sums the two output signals from the DAC and a single-ended buffer (in
class-A) at the output. All of this is entirely built on the PCB of the
player, by using the original lay-out.

The passive filter is optimized to make the group-delay in the passband
as constant as possible (Bessel characteristic). The price for this is
an attenuation of the signal of 1dB at 20kHz. Accurate
1% polystyrene capacitors and
0.1% resistors are used. The 10mH inductors are matched by hand with an
accurate LCR meter. This insures both filter curves for the left and
the right channel are as identical as possible.

In the output stage, either JFETs
and transistors
can be used, or both. The 2SK170
and BC550 that are used here are
pin-compatible: their pins D-G-S and C-B-E match by functionality. Both
versions were built and
sound very good.
Needless to say this also depends on the quality of the components
used. The output stage was
later upgraded with Holco and PRP resistors, and a Multicap tin-foil
output capacitor.

Here
only the photos of this specific section are presented, the rest of
the modifications are similar to those of the CD67mkII-OSE.

During the modification of the player, the
muting transistors are also removed. These are connected between the
analog outputs and ground (through a 100Ω resistor) and make sure that
the annoying noises that sometimes occur during power-up or -down, or
during errors on the CD, are being suppressed. However, the properties
of a transistor are voltage and current dependent, and considering the
fact the output signal is very dynamic, it's better not to use any
semiconductors in this section. In the modification list, the
application of a relay is mentioned, to restore the muting function.
The contacts of this relay come in place of the transistors, and will
short the outputs to ground if no music is played. The power-up and
down noise will be a thing of the past with this.
The supply voltage for the relay will be tapped from the collector of
QN02. An auxilary voltage that is originally used for muting is present
there. It is about 12V, which is
perfectly suited for driving a relay. A double-pole/double throw (DPDT)
12V relay with gold-plated contacts is used here. It's best to use one
that is specifically
suited for switching small signals, for instance the G5A-234P-12VDC
from Omron. It
is also possible to use two single-pole relays. Two 6V relays can be
connected in series, so only one transistor is needed to drive them. If
RN30 and RN31 are lowered to 2k2, the original muting traces on the PCB
can be used to drive T1. QN24 and QN25 have to be removed, and their
base and collector connections have to be jumpered on the PCB. This is
needed to connect the muting signal that's coming from the decoder
(Q102). RN27 and RN28 have to be removed as well. Connect the base of
T1 to the
PCB, to the base of either QN07 or QN08.

The two NC (normally-closed) contacts of the relay are connected
between GND (the collectors) and the emitters of QN07 and QN08. Make
sure the output signal is not shorted to GND directly. A resistor of a
few tens of ohms (33...47Ω) has to be connected in series with the
output signal, before the relay contacts, in the form of R659/660.